The present invention relates to a screw fastener for the fastening of non-metallic sheeting to a support. The invention has been developed principally for connecting fibre cement sheeting or the like, to metal battens or light gauge studs and it will therefore be convenient to describe the invention hereafter with reference to that particular application. It should however be appreciated that the invention has wider application and can for example, be applied to the application of fibre cement sheeting to a timber support.
Fibre cement sheeting is commonly secured to metal battens with self-tapping threaded fasteners of the kind having a drilling tip formed at one end of a shank and a head formed at the opposite end. The screw fasteners are normally driven into the fibre cement sheeting with a powered driver and it is a drawback of the present fasteners, that the screw head often is driven to a position either proud of, or well below the outer or visible surface of the sheeting and is therefore not flush with that surface. This occurs because there is insufficient control relating to the penetration depth of the screw into the sheeting and generally occurs regardless of the experience of the operator.
No practical solution has been heretofor found to the above problem, which affects the finished appearance of the sheeting. In this respect, the sheeting surface is normally finished by painting, or by other suitable coverings, after it has been secured in place against the support and the existence of a screw head proud of the sheeting surface detracts from the surface finish and/or prevents proper attachment of suitable coverings thereto. Therefore, each head sitting proud of the sheeting surface has to be ground back to be flush with the sheeting surface. The existence of a screw head well below the sheeting surface leaves an exposed bore in the sheeting which has to be filled, such as with grout or putty, prior to finishing the sheeting surface. Such additional treatment described above is tedious and adds to the labour costs involved in the floor, ceiling or wall construction, and also delays completion of the construction. The existence of a screw head well below the sheeting surface also can cause damage to the fibre cement sheeting, such as by delamination, which requires additional repair.
It is an object of the present invention to provide a screw fastener that overcomes or at least alleviates the above disadvantages of known screws. It is a particular object of the invention to provide a screw fastener that provides a level of control in relation to the depth of penetration of the screw into a sheeting substrate.
According to the present invention there is provided a screw fastener including an elongate shank having a longitudinal axis, a tip at one end of said shank, a head at the opposite end of said shank, and a self-tapping screw thread extending along at least a major portion of said shank, said head including a generally planar end face which is generally perpendicular to said longitudinal axis, and which includes means for cooperation with a screw driver, said head further including boring means that permits said head to bore into a sheeting substrate and control means for controlling the depth of penetration of said head into the sheeting substrate.
The control means can take any suitable form, but preferably is operative by increasing the torque required to drive the screw, to a level that causes the screw driver to cease driving the screw. This may be achieved in one form, in an arrangement in which the boring means includes a plurality of cutting edges disposed about the longitudinal axis of the screw fastener. The cutting edges extend at an angle from at or adjacent the shank in a direction away from the tip, to at or adjacent the outer edge periphery of the head and are separated by a flute or channel or the like, that extends towards the end face of the head, for removal of bored material (otherwise known as xe2x80x9cswarfxe2x80x9d). The head defines a control lip that extends between at least a pair of cutting edges and which prevents or at least substantially limits removal of bored sheeting material from between the pair of cutting edges, when the end face of the head reaches a position in which it is adjacent to or substantially flush with the outer surface of the sheeting. This is achieved by the control lip closely approaching or engaging the wall of the bored opening so as to substantially or fully close the passage between one or more pairs of cutting edges through which bored material is otherwise removed from the bored opening.
By preventing or substantially limiting removal of bored material from between a pair of adjacent cutting edges, that material is largely captured within the bore hole and it resists further penetration of the screw head into the sheeting material. Thus, the control lip bears against and compresses the bored material within the opening in the sheeting made by the boring means. That resistance to further penetration results in a greater torque being required to be applied to turn the screw, and the arrangement can be such that the increased torque reaches a level at which further drive of the fastener is prevented.
In the above arrangement, the screw driver can be of a known drill-type, with a screw head adaptor provided to cooperate with the head of the fastener to turn the fastener and upon reaching the increased torque level described above, the drill can either jam or the adaptor be released from engagement with the head. Alternatively, the screw driver may have a torque limiter that stops the driver from driving above a certain torque level.
In the above arrangement, the penetration of the head into the sheeting material can be controlled to suddenly stop penetration, advantageously providing accuracy and reliability in relation to the extent of head penetration. This is facilitated by the action of the fastener thread engaging the metal batten and requiring the fastener to penetrate the sheeting at a certain speed. That is, threaded engagement of the fastener with the batten constrains the fastener to move axially at a speed determined by the thread helix angle and the rotational speed of the drive. When axial penetration of the fastener into the sheeting is resisted by obstruction of the head, initially, any gap existing between the sheeting and the batten will be removed by the fastener drawing the sheeting and the batten together, whereafter the torque required to drive the fastener will rapidly, and in the preferred arrangement, effectively instantly increase.
The control lip of the invention preferably has a planar face that faces in the axial direction of the fastener toward the tip. The control lip preferably is formed on the undersurface of the end face of the head and preferably the edge periphery of the control lip is formed by the outer edge periphery of the head.
A control lip may extend between only a single pair of cutting edges, but preferably a control lip extends between more than a single pair and most preferably between each pair of cutting edges. In a preferred arrangement, the boring means includes at least four cutting edges, but preferably six and the control means therefore preferably includes respectively four or six control lips.
The or each control lip can be axially spaced from the end face any suitable distance to ensure that in use, the end face is positioned substantially flush with the sheeting surface when the control means operates to prevent further penetration of the head into the sheeting material. The spacing will take into account any further travel of the head into the sheeting, once the or each control lip is operative to commence restriction on further penetration of the head into the sheeting material, such as when the control lip has entered or is adjacent the mouth of the bored opening. That further travel may occur as the bored material is compressed by the control lip into the bored opening. In one arrangement, the spacing between the end face and the control lip is in the region of less than 1 mm.
In an alternative embodiment, the radial extent of the cutting edges may be less than the radial extent of the outer edge periphery of the end face and the control means may be formed by an underneath surface of the head opposite the end face that extends radially beyond the cutting edges. In this arrangement, the head will penetrate into the sheeting material, boring an opening of a radius equal to the maximum radial extent of the cutting edges. The control means will thereafter limit penetration of the head into the sheeting material, by engaging the surface of the sheeting material around the bore opening.
In the above arrangement, engagement of the control means with the surface of the sheeting material under a driving force will cause the sheeting material to compress and with a suitable material, the level of compression will be sufficient to embed the head in the material so that the end face thereof is substantially flush with the sheeting surface. This arrangement is not however suitable for all types of sheeting material, in particular those having low levels of compressibility.
The invention advantageously facilitates accurate positioning of the end face of the fastener head substantially flush with the outer surface of the sheeting material, so that there is no requirement for further treatment of the sheeting surface by grinding or filling, although in the event that the sheeting surface is to remain visible, i.e. it is not to be covered after fixing to the batten or stud, it may be necessary, or just desirable to fill any slight depression caused by compression of the sheet material by the screw fastener. The screw fastener of the invention advantageously requires no special skills to achieve the desired flush installation and can be installed with equipment presently used to install known screw fasteners. It is therefore considered that the invention provides significant advantages over such known screw fasteners.